1. Field of the Invention
The present invention relates to the field of power supplies and other electronic circuits wherein overshoots from transient conditions causing slewing of one or more circuit nodes need to be controlled.
2. Background of the Invention
In many power supply circuits, it is desirable to limit output voltage overshoot, at startup or under various transient conditions, to very low values that might typically be say less than about 2%. This often places severe restrictions on the design of their control circuits with regard to the AC performance of the embedded error amplifiers and the associated feedback loops. In order to achieve very low values of output voltage overshoot, the conventional approach is to use more current (translates into more power) in the biasing of the error amplifiers and to restrict the range of output loads (usually a combination of dissipative elements and capacitors). It should be noted that under normal operating conditions the error amplifier is assumed to be functioning in a linear mode and, of course, must be stable. However, there is a second mode whereby for large transient conditions (for example load changes, input supply voltage changes, etc.), the error amplifier may be required to slew part or all of its circuitry to respond to the transient. During slewing the error amplifier does not operate in a linear manner whereby its output is simply defined by its small signal, low frequency characteristics and feedback loop(s). Instead, the rate of change of voltage at the output, and maybe also at one or more nodes within the error amplifier, will be limited by the available values of the drive current and the associated component and parasitic capacitances at these nodes. Consequently, large operating transients are not responded to well by the control circuitry, and overshoots can occur because of the finite time that the control circuit takes to recover from a slew mode to a linear mode of operation. The invention describes a technique to limit voltage overshoot without compromising the control circuitry nor requiring the use of high biasing currents, output load restrictions, etc. Also, to the first approximation, the technique of the present invention does not affect the control circuit loop stabilities or small signal response of the circuitry.
Also most analog systems including amplifiers, signal conditioners etc. have high frequency limitations. One limitation occurs when an input signal has a rate of change of voltage greater than a specific value above which the rate of change of the output signal does not further increase. Under these conditions the system is said to be slewing--a further increase of rate of change of input voltage is not reflected at the output as a further increase of rate of change of output voltage. The reason for slewing is due the finite values of capacitance (both component and parasitic) at each circuit node and the finite available nodal drive currents. The available nodal drive current divided by the finite value of capacitance of the node defines a maximum nodal rate of change of voltage or nodal slew rate. When a system, having one or more signal path nodes (there must be at least one node--the output node) slews, it is caused by the limitation of the maximum rate of change of voltage of the slowest node.
However, if the system is operating in a low frequency linear mode, each node assumes voltage values primarily dependent upon the system transfer characteristics, the input driver parameters and the output load parameters. Nodal voltages do not depend, to the first order, on nodal capacitances nor nodal drive currents. Consequently, many analog systems may be made to operate in two very different modes--linear and slewing.
An ideal system will respond to a transient (output load change, power supply glitch, etc.) in a benign way; namely, without overshooting. In the real world, compromises are made to minimize overshooting that can be undesirable. These may include higher nodal current drives, restrictions on output loads, reduced system gains (often resulting in lower system accuracies), and so on. Voltage overshoots are often the consequence of differential nodal slewing whereby the effective slew rate of certain node(s) are faster than other nodes. Consider the example shown in FIG. 1a which depicts a simple operational amplifier operating in a unity gain mode having a differential input stage with an output at node VA. This output drives a single ended gain stage with a load ZL. (Both the differential input stage and the single ended gain stage are inverting stages in this example.) It is assumed that the circuit as shown is small signal stable having internal frequency compensation (this will be described later) and that node VA can slew faster than the output node can with its applied load ZL. FIG. 1b shows the response of node VA and the output node to a moderate amplitude square wave input signal. Note that the voltage at node VA rapidly slews to its maximum deviation value well before the output signal has reached its desired output level. When the output signal reaches its desired output level the voltage at node VA has to slew back to the DC level necessary to control the output voltage level to its desired level. However this takes a finite time and so for that time the second stage has an excess drive on its input and consequently the output overshoots. In the example, for simplicity, the overshoot is shown damped and returning after the overshoot to its correct voltage level. More often in practice the overshoot, instead of being critically damped, will exhibit a damped ringing response shown by the dotted curves in FIG. 1b.